petMAP PETMAP+ II - Measuring SpO2; Requirements for Obtaining Reliable SpO2 Readings

33
Measuring SpO2
Theory of Operation
The pulse oximetry function of petMAP+II determines both pulse rate and SpO2 % (percent oxygen
saturation) by passing two different wavelengths of low intensity light (one is a visible red and the other
is an invisible infrared) through body tissue which has good blood flow to a photodetector. The sensor is
divided into two halves: the emitter is the part of the sensor that glows red when powered on; the
detector is the opposite part of the sensor and does not emit any light. SpO2 values are based on
calculations of the ratios of the differences in absorbance of red and infra-red light between blood
saturated with oxygen and oxygen-depleted blood at each heartbeat. The variation of the absorbance
of the infrared light during each heart beat is displayed as the O2 pleth trace.
Requirements for Obtaining Reliable SpO2 Readings
Proper Sensor Placement: The user must place the sensor clip on a relatively thin tissue that has good
circulation, such as the tongue, lip, deep in the ear, paw, toe, toe webbing, arm or leg just above the
paw, vulva, or Achilles tendon. The tissue being used must be of a thickness and color that allows
proper light penetration of the tissue between the two halves of the sensor clip. Darkly pigmented skin
or dark fur can prevent proper light penetration and thus prevent O2 measurement. Note that the
sensor should NOT be placed on an extremity that has a blood pressure cuff, arterial catheter or
intravascular line proximal to the O2 sensor site.
Proper Sensor clamping pressure: If needed and so equipped, the sensor’s clamping pressure
adjustment should be used to reduce sensor clamping pressure so that the pressure of the sensor does
not compress the tissue and prevent good circulation in the tissue between the sensor’s two arms. This
pressure reduction adjustment is made by using the grey pressure reduction slider (details below) and is
often needed when measuring on the ear and tongue of small animals. Specifically, the pressure can be
relieved by moving the gray strain relieve slider towards the sensor so that clamping tension is placed on
the rear portion of the clip by the two sensor wires. This pressure on the rear portion of the sensor
reduces the pressure on the sensor jaws which will prevent the sensor pressure from cutting off the
circulation in the tissue between the sensor jaws. However, care should be taken to maintain just
enough pressure to hold the sensor in place. (see lower left photo below which shows the slider
snugged up close to the sensor to relieve pressure on the ear)
No subject motion: The animal must be motionless. Any motion will affect the stability and accuracy of
the readings, and continuous motion such as shivering will prevent accurate measurement. Generally,
SpO2 readings are best taken on anesthetized patients or patients that are not moving.
Reset O2 electronics: After any change in the sensor positioning, the O2sat ON/OFF button should be
cycled OFF and then back to ON. This allows the O2 electronics to restart using the new sensor/tissue
setup. It will typically take 15-20 seconds to get a first reading and up to 30 seconds for the readings to
stabilize. During monitoring, disregard the SpO2 readings if the pleth trace is not stable and/or if the
patient’s heart rate is not reasonably correct. Either of these conditions requires sensor adjustment and
resetting the O2 switch to OFF, then ON.